In the search for suitable materials from which to construct high energy density solid state batteries, one of the principal obstacles has been the provision of a suitable electrolyte. A variety of approaches have been tried heretofore. The one which received the most attention among those prior approaches is the one based on polymer solvents in which an optimized amount of ionic salt is dissolved in the polymer solvent (See Armand et al., U.S. Pat. No. 4,303,748; Andre et al., U.S. Pat. No. 4,357,401; and Kronfli et al., U.S. Pat. No. 5,009,970). Other approaches, which possessed both specific advantages and disadvantages, involved glassy solid electrolytes, and certain plastic crystal or disordered crystal electrolytes. Neither of these approaches, nor any of the prior art approaches, obtain all the properties generally required of an electrolyte for the successful development of a high power solid state battery, namely: (1) high ionic conductivity at desired use temperatures preferably between about -20.degree. C. to +100.degree. C. (with conductivities of about 10.sup.-3 S/cm or above at room temperature); (2) conductivity predominately by cations (to avoid undesirable cell polarization problems); (3) a rubbery or viscoelastic consistency (to permit the deformation of the electrolyte as needed to accommodate volume changes during charging and discharging cycles); (4) stability over a wide electrochemical potential range (to permit the utilization of anode/cathode combinations which provide high voltages); and (5) good adherence to the electrode surfaces (to prevent mechanical/electrical problems that could other-wise develop during charging and discharging cycles).
Each substance heretofore developed for solid electrolyte purposes possesses only a limited number of the above-identified desiderata. None achieved them all. For instance, the so-called superionic glass electrolyte, exemplified in the most successful case by Li.sub.2 S--LiI--Y (where Y is a Lewis acid such as P.sub.2 S.sub.5 B.sub.2 S.sub.3.SiS.sub.3) achieves some of the above listed properties namely, 1, 2, 4 and 5, but is very brittle and totally lacks the desired rubbery or viscoelastic consistency. Examples of this type of electrolyte are described by Malugani et al. in U.S. Pat. No. 4,331,750 and by Akridge in U.S. Pat. No. 4,585,714.
The prior art salt-in-polymer approach mentioned above-satisfies three of the desiderata namely, 3, 4 and 5, but fails miserably with regard to desired properties 1 and 2. For instance, neither of two recent U.S. Patents dealing with salt-in-polymer electrolytes reported a room temperature conductivity greater than 10.sup.-5 S/cm for solvent-free or plasticizer-free systems (See: Kronfli et al., U.S. Pat. No. 5,009,970; Knight et al. U.S. Pat. No. 4,737,422). One prior art effort to rectify the poor conductivity of the salt-in-polymer electrolyte involved the addition of low molecular weight plasticizers (solvents) to the mixture (See: Koksbang et al. J. Power Sources 32, 175, (1990)). Improved conductivity was achieved, but at the expense of introducing unwanted volatile components into the electrolyte making the electrolyte susceptible to composition changes when it is exposed to the external atmosphere. Since the solubility of lithium salts in the polymer electrolytes is predicated upon attraction between the lithium cations and the solvating groups in the polymer, these electrolytes further suffer from the fact that the lithium is the less mobile cation. This means that the cation conductivity desideratum, identified as "2" above, is never achieved except in the poorly conducting, single mobile ion polymers which are described by Noda et al. in U.S. Pat. No. 4,844,995. It is believed that it is fundamentally unlikely that this problem can be rectified with the usual salt-in-polymer type of medium. Claims have been made that the problem can be somewhat reduced by using plasticized polymers, although no verification of these claims has been found. Exemplary salt-in-polymer type electrolytes are disclosed in U.S. Pat. Nos. 4,303,748; 4,357,401; 4,585,714; and 5,009,970.
U.S. Pat. No. 4,234,667 of Bennion et al. discloses molten salt electrolytes of lithium chlorate or lithium perchlorate singly or in or in combination with lithium chloride and lithium oxide, as useful electrolytes in high temperature cells. Such electrolytes provide useful conductivities only in the range of 140-160.degree. C., or higher. Owing to the very low ionic conductivities, battery cells comprising said electrolytes are essentially inoperable at temperatures below 140.degree. C. and thus, can be stored for lengthy periods at room temperature without said cells suffering appreciable loss of effectiveness. Thus, said materials disclosed in U.S. Pat. No. 4,234,667 do not satisfy desideratum identified as #1 above.
Ambient temperature molten salt electrolytes containing Li salts are described by Cooper and Angell, Solid State Ionics, 9 & 10, 617 (1983), and by Takami et al., (Chem Abs 118: 195131d, Jpn. Kokai Tokkyo Koho JP 04,349,365 92,349,365! CI. HOIM10/40.03 December 1992. Appl. 91/120,836. May 27, 1991: 4 pp). Neither of the latter electrolytes fall within the scope of the present application because they owe their high room temperature conductivities to the lowering of T.sub.g obtained by the inclusion of considerable mole fractions of non-lithium salts, namely, organic salts (tetraalkyl ammonium or otherwise-substituted ammonium salts). Inclusion of such organic salts destroys the decoupling of the Li.sup.+ ion motion, hence leads to undesirably low Li.sup.+ transport numbers.
Also, U.S. Pat. No. 4,164,610 discloses essentially silica-free glasses which have compositions within the Li.sub.2 O--Al.sub.2 O.sub.3 and/or F--B.sub.2 O.sub.3 system. These glasses exhibit high lithium ion mobility at 200.degree. C. but low mobility at room temperature (10.sup.-7.57 S/cm at best) according to Table II of U.S. Pat. No. 4,164,610.
As is apparent, a great need exists for the development of an improved electrolyte which obtains all of the desiderata listed above without the acquisition of unacceptable deleterious properties. While superionic glass electrolytes have been heretofore known to obtain various important properties such as high Li.sup.+ conductivity, this so-called superionic glass electrolyte is brittle. Furthermore, the rubbery salt-in-polymer alternatives, which have sufficient resiliency to absorb electrolyte stress, suffer from being predominantly anion conductors. None of the known electrolytes achieves the unique combination of properties, so long desired but heretofore unobtainable, that is achieved by the present invention as is hereinafter described in detail.
Accordingly, a principal object of the present invention is to provide new and improved alkali metal ion conducting electrolytes that provide unexpectedly high conductivities both at room temperature and at 100.degree. C.
Another object is to provide new and improved predominantly Li.sup.+ -conducting solvent-free viscous liquid electrolytes that provide conductivities of almost 10.sup.-2 S/cm at room temperature and almost 10.sup.-1 S/cm at 100.degree. C.
Still another object of the present invention is to provide a new and improved solvent free alkali metal ion conducting electrolyte containing molten alkali metal salts and having a long-chain high molecular weight neutral or anionic polymer dissolved therein to provide a rubbery, non-brittle consistency to the electrolytes even at low temperatures without sacrificing the high conductivity obtained thereby.